Lead-acid batteries have been widely applied in automobile, UPS, and mobile communication equipment fields, etc., owing to their advantages of low price, high safety, and reliable techniques, etc. Moreover, as the automobile consumption in China grows rapidly in recent years and the European Union promotes BlueMotion automobiles that employ an auto start/stop technique, it is foreseeable that the production of lead-acid batteries will still grow persistently in a long time in the future. Statistical results demonstrate that the yield of refined lead in China in 2012 was as high as 4.646 million tons, in which about 3.3 million tons of refined lead was used to manufacture lead-acid batteries. As primary lead ore resources become deficient gradually, waste lead-acid batteries have become the principal raw material in the regenerated lead industry increasingly. How to achieve clean recovery of waste lead-acid batteries is not only a demand for environmental safety, but also an urgent mission to be fulfilled for sustainable development of the regenerated lead industry.
At present, waste lead-acid batteries mainly contain waste lead plate gratings and lead paste, wherein, the lead paste accounts for about 60-65% of the lead used for lead-acid batteries. Compared with waste lead plate gratings, from which lead can be recovered simply by direct smelting, waste lead paste contains lead and lead compounds in a variety of forms, usually including Pb (8-13 wt. %), PbSO4 (35-40 wt. %), PbO (8-15 wt. %) and PbO2 (35-40 wt. %), and is the main challenge in the present lead recovery work. For a long time, the traditional pyrometallurgical recovery process has been used in the modern lead recovery industry. To decrease the smelting temperature and reduce emission of sulfur dioxide in the waste lead paste smelting process, sodium carbonate, ammonia or sodium hydroxide is usually utilized to remove lead salts (e.g., lead sulfate) in the lead paste in advance before the smelting procedure in the modern lead recovery process. Since lead sulfate is the principal lead salt in lead paste, the pre-desalting procedure of lead paste is usually referred to as a desulphurization procedure, i.e., utilizing an alkaline substance to remove the lead sulfate component in the lead paste. After desulphurization, the lead paste has to be reduced at a high temperature to obtain crude lead. Owing to the fact that the lead content in crude lead is usually 97-99%, crude lead has to be refined by electrolytic refining or pyro-refining to obtain refined lead at 99.99% purity, before it can be accepted by the downstream lead-acid battery manufacturers. Actually, refined lead cannot be used directly as an active substance in the manufacturing of lead-acid batteries; instead, refined lead has to be treated through three main procedures, i.e., melting, casting into lead balls, and ball milling and oxidation, to obtain lead oxide, which can be used as the principal raw material for producing lead paste. The lead recovery process involves a pyrometallurgical process at a temperature as high as 1,300° C., which not only consumes energy heavily, but also generates a large quantity of lead-containing waste residue and lead-containing dust in particle size smaller than PM2.5, resulting in severe secondary lead pollution is some regions. Likewise, the refining process of crude lead and the ball milling and oxidation process of refined lead also consume electric power heavily, and generate a large quantity of lead-containing dust. Apparently, the existing lead recovery process requires several links that consume energy heavily, including high temperature smelting, electrolytic refining, lead ingot melting, and ball milling and oxidation, etc., and is the main reason for heavy energy consumption and pollution of the present lead recovery industry, in which a pyrometallurgical process is employed. To meet the demand for lead oxide in the production of lead-acid batteries, it is desirable to invent a novel process for directly converting lead paste in waste lead-acid batteries or pre-desalted lead paste into high purity lead oxide required in the production of lead-acid battery, which will be a clean, energy-saving, and short-flow process.
Some research groups have made beneficial trials on conversion of waste lead paste into lead oxide. For example, as disclosed in the Chinese patent application No. CN201210121636.2, a raw material such as sodium carbonate is utilized to have a desulphurization reaction with waste lead paste, and the obtained desulfurized lead paste has a reaction with citric acid solution, and then the reaction product is dried to obtain lead citrate; finally, the lead citrate is calcined to prepare super-fine lead oxide. Though the target product in that invention is PbO, large quantities of chemical raw materials, including citric acid, hydrogen peroxide, and sodium carbonate, etc., are consumed. Therefore, that process is uneconomical from the viewpoint of atom utilization.
The concept of atom economy was put forth initially by Professor B. M. Trost in 1991 to improve the productivity of organic chemistry, in the hope that atom economy should be considered in organic synthesis reactions to convert the atoms in raw materials into target products as far as possible. Though the concept of atom economy was put forth against a phenomenon that there are many subsidiary reactions and the yield is low in organic chemical processes, actually a severe problem of atom economy also exists in traditional inorganic chemical processes in the modern times. For example, an existing wet process for recovering lead oxide from lead oxide ores mainly comprises the following 4 steps:    (1) first, dissolving PbO ores in nitric acid to obtain lead nitrate; in that step, other metal oxides are also dissolved in the nitric acid;    (2) adding sulfuric acid into the nitric acid dissolving solution, to obtain lead sulfate precipitate and waste nitric acid that contain other metals;    (3) filtering and separating, to obtain lead sulfate, and then adding sodium carbonate into the lead sulfate for desulphurization, to obtain lead carbonate and waste sodium sulfate solution;    (4) heating up the lead carbonate, so that the lead carbonate is decomposed to obtain a PbO product, and carbon dioxide waste gas is emitted.
The reaction equation and atomic quantities in the reaction process are expressed as follows:PbO+2HNO3═Pb(NO3)2+H2O+10 atoms  (1)Pb(NO3)2+H2SO4═PbSO4+2HNO3+7 atoms  (2)PbSO4+Na2CO3═PbCO3+Na2SO4+6 atoms  (3)PbCO3═PbO+CO2  (4)
In the above process that comprises 4 steps, the initial raw material PbO is composed of two atoms Pb and O, wherein, the reactant atoms added in the reactions expressed by the reaction equations (1), (2) and (3) are 2HNO3 (10 atoms), H2SO4 (7 atoms), and Na2CO3 (6 atoms) respectively; thus, the quantity of atoms participating in the entire process is 2+10+7+6=25, and the quantity of atoms in the target product is 2; hence, the atom utilization ratio in the entire process is 2/25=8%.
Apparently, the atom utilization ratio in the traditional lead oxide recovery process that seems reasonable is only 8%, which means that a large quantity of atoms from different raw materials is wasted in the production process. Consequently, that process not only consume raw materials heavily and brings a problem of high production cost, but also produces wastes, including waste nitric acid, waste sodium sulfate, and waste carbon dioxide, which may pollute the environment. Therefore, developing a novel atom-economic reaction or a process that has a high atom utilization ratio is the principal solution to clean recovery of waste lead materials.
A research group led by Professor Junqing Pan of Beijing University of Chemical Technology has done a lot of work related with lead-acid battery recovery. However, the research made by the research group in the early stage did not follow the principle of atom-economic reaction. For example, the authorized patent ZL201010297522.4 of the research group followed the traditional ideal in foreign countries, i.e., converting all other components (Pb, PbO and PbO2) of lead paste into lead sulfate in sulfuric acid; consequently, the lead paste that contains 35-40% lead sulfate originally is turned into 100% lead sulfate though the sulfating reaction, resulting in consumption of a large quantity of sulfuric acid; moreover, additional 60-65% NaOH has to be consumed in the follow-up desulphurization stage to remove the newly added lead sulfate; as a result, not only the conversion cost is increased, but also a large quantity of atoms is wasted. A similar patent (Chinese Patent No. CN201310250004.0) has set forth a wet recovery method for lead-containing raw materials, wherein, the waste lead paste in lead-acid batteries is converted into lead sulfate and basic lead sulfate, but that method also has a problem of raw material consumption. Then, in a combined electrolytic lead recovery process (authorized patent ZL201110293590.8), the problem of atom economy was considered, and different desulphurization and stepwise electrolytic reduction measures were used for PbSO4 and PbO2, and thereby the problem of chemical raw material consumption in the preceding process was solved to a great extent; however, that process was still in the realm of the traditional metallic lead recovery ideal. In later work of the research group, such as the patent CN201210535154.1, new research was made to improve the utilization of atom economy in the conversion process, and the target product was direct a PbO product. In that work, NaOH was used to process lead paste to obtain sodium sulfate and NaHPbO2 solution, then the solution was cooled to obtain crude PbO, and the crude PbO was dissolved again in NaOH solution for recrystallization, so that pure PbO was obtained. We have found a problem when we practicing the method disclosed in the patent: the Pb and PbO2 reaction velocity in NaOH solution is low, and excessive PbO2 and NaHPbO2 may have subsidiary reaction to generate Pb3O4 that is stable in thermodynamics, as indicated below:Pb+PbO2+2NaOH═2NaHPbO2 PbO2+2NaHPbO2═Pb3O4+2NaOH
To solve the above problem, the latest patent document CN201310084392.X has disclosed a method that directly utilizes a high-temperature solid-phase atom-economic reaction to obtain lead oxide from waste lead paste. In that work, waste lead paste and lead powder are heated up to have a solid phase reaction; then the mixture is desulfurized with a first sodium hydroxide solution to obtain crude PbO; next, the crude PbO is leached with a second sodium hydroxide solution to obtain lead-containing alkaline solution and filter residue; next, the lead-containing alkaline solution is purified and cooled to crystalize, so that PbO is obtained. Finally, the PbO is dissolved again in a third sodium hydroxide solution for recrystallization to obtain high-purity PbO crystals; then, NaOH is replenished to the first sodium hydroxide solution after desulphurization so that sodium sulfate crystals precipitate; thus, a NaOH desulphurization cycle is established, and a byproduct sodium sulfate is obtained. A characteristic of that method is: crude PbO is obtained through a solid phase reaction between Pb and PbO2 in waste lead paste at a high temperature, the high temperature condition greatly improves the reaction velocity between Pb and PbO2, and the residual PbSO4 part is desulfurized with a first sodium hydroxide solution to obtain PbO; finally, the two PbO parts from different sources are dissolved together in NaOH solution, and purer solid PbO is obtained through a recrystallization process. In that method, only the desulphurization of the PbSO4 part in waste lead paste consumes NaOH, while the atom-economic reaction between Pb and PbO2 in lead paste and the reversible dissolution-crystallization process of PbO in NaOH are zero-consumption atom-economic processes theoretically. After research for almost one year, it is found that the method still has the following drawbacks at present:                1. Three NaOH solutions and two recrystallization processes are involved in the recovery process to obtain high-purity PbO;        2. In the initial solid phase conversion stage of waste lead paste at a high temperature, the PbSO4, which accounts for 30-45 wt % of the lead paste, doesn't participate in the reaction before and after heating process; consequently, a lot of heat is consumed in vain, and the solid phase reaction between Pb and PbO2 is not complete because a great deal of lead sulfate is included in the lead paste;        3. Owing to the fact that the lead paste contains a variety of impurities, such as Sn, Al and Sb, etc., which comes from the alloy of the plate grating, these amphoteric metals will be dissolved by the NaOH solution; consequently, the NaOH mother liquid is gradually contaminated by the impurities in the recrystallization process.        
Therefore, it is a challenging and urgent task in the field of recovering waste lead-acid batteries currently to develop a novel process that can directly recover PbO from waste lead paste and has an atom-economic characteristic, in order to solve the problem that it is difficult to separate amphoteric metals with the current NaOH system and inertial Pb3O4 may be generated easily.